The present invention relates to a process for treating a dioxin-containing exhaust gas and a composite catalyst composition for inhibiting the generation of dioxin, and more particularly, to a process for treating a dioxin-containing exhaust gas discharged from intermittent operation-type solid waste incineration facilities such as mechanical batch incinerators or semi-continuous incinerators as well as continuous operation-type solid waste incineration facilities, and a composite catalyst composition for inhibiting the generation of dioxin. Still more particularly, the present invention relates to a process for treating a dioxin-containing exhaust gas which can inhibit the generation of dioxin occurring either in the course of cooling of the exhaust gas between a downstream portion subsequent to a combustion chamber and a dust collector or due to a memory effect upon low-temperature combustion in flues or dust collector at the start-up or shut-down of intermittent operation-type solid waste incineration facilities, without large-scale incineration facilities or plant renovation and equipment investment, and a composite catalyst composition for inhibiting the generation of dioxin.
Upon waste disposal, various wastes such as municipal solid wastes or the like have been divided into reusable ones and the others. After the reusable ones are recovered as effective resources, remaining combustible wastes have been usually incinerated. Incinerators used for waste disposal are classified into four types according to its operation time a day (i.e., into mechanical batch incinerators, fixed batch incinerators, semi-continuous incinerators and full-continuous incinerators). In Japan, about 24% of these incinerators are of a large-scale continuous operation type. Whereas the waste incineration has been still conducted using intermittent operation-type incineration facilities such as mechanical batch incinerators or semi-continuous incinerators.
At present, dioxin contained in exhaust gas discharged from waste incineration facilities causes significant social problems. It is suggested that the dioxin is generated due to incomplete combustion in a combustion chamber or in the course of cooling between a downstream portion subsequent to the combustion chamber and a dust collector. The dioxin is extremely toxic and is considered to promote a carcinogenesis. Also, the dioxin is undecomposable in natural environment, and therefore, when being taken into human bodies through water or foods, the dioxin is accumulated within the human bodies. For these reasons, it is necessary to reduce the amount of dioxin generated to as small a level as possible.
In the continuous operation-type solid waste incineration facilities, wastes can be continuously incinerated in a combustion chamber thereof at an elevated temperature and, therefore, are readily subjected to complete combustion, so that the amount of dioxin generated in the combustion chamber can be limited to a low level. However, it has been reported that dioxin is generated in the course of cooling of exhaust gas between a downstream portion subsequent to the combustion chamber and a dust collector due to a so-called xe2x80x9cde novoxe2x80x9d synthesis (H. Huang et al., xe2x80x9cChemospherexe2x80x9d, 31, 4099(1995)). Even the continuous operation-type solid waste incineration facilities still fail to prevent the generation of dioxin in the course of cooling of the exhaust gas between a downstream portion subsequent to the combustion chamber and the dust collector to a sufficient extent.
Also, in the intermittent operation-type incineration facilities, dioxin tends to be generated in the course of cooling of the exhaust gas between a downstream portion subsequent to the combustion chamber and the dust collector like the continuous operation-type ones.
In particular, the intermittent operation-type incineration facilities are operated at predetermined time intervals. Accordingly, it takes several hours from the start-up until the incinerator can be operated in a steady state. At every start-up operation, there is caused a low-temperature combustion condition in which the dioxin tends to be generated. Also, when the incinerator is stopped, a part of the solid wastes is continued to smolder while causing incomplete combustion thereof. In such a case, when the incinerator is started up again, unburned substances caused due to incomplete combustion upon the shut-down of the incinerator on the previous day and upon the start-up thereof on the next day, still remain inside flues or dust collector of the incinerator. In consequence, there has been pointed out a disadvantage that such a so-called memory phenomenon that dioxin is re-synthesized and then discharged into an exhaust gas even though the temperature of the exhaust gas is as low as not more than 200xc2x0 C., is caused and continued for several hours after the start-up of the incinerator (refer to Kawakami, Mtsuzawa and Tanaka, xe2x80x9cProceeding of the 5th Annual Conference of Japan Society of Waste Management Expertsxe2x80x9d, p. 264 (1994)). Thus, it has been reported that the intermittent operation-type incineration facilities generate a larger amount of dioxin as compared to that of continuous operation-type ones.
The amount of dioxin generated from waste incinerators has been legally strictly limited. The Ministry of Health and Welfare has planned to replace these intermittent operation-type incineration facilities with continuous operation-type ones hereafter. Further, according to xe2x80x9cthe Air Pollution Control Actxe2x80x9d enforced on Dec. 1, 1997, the upper limit of amount of dioxin discharged from incinerators has been regulated. Unless the upper limit (new incinerators: 0.1 to 5 ngTEQ/Nm3, existing incinerators: 1 to 10 ngTEQ/Nm3, according to scales of incineration facilities) is reached within 5 years henceforth, the deficient incinerators must be scrapped.
However, due to the financial problems of local governments, etc., it is difficult to scrap the existing incinerators and construct news incinerators instead. For this reason, there has been studied a method of overcoming the regulation while continuously using the existing incinerators.
Therefore, it has been strongly required to provide a process for treating an exhaust gas so as to prevent the generation of dioxin without a large-scale incineration facilities or plant renovation and equipment investment.
Hitherto, as the prior arts relating to absorption or decomposition of dioxin generated upon incineration of the wastes, various methods have been reported. For example, there are known a method of decomposing poly-halogenated aromatic compounds having at least five carbon atoms by heating at a temperature of 200 to 550xc2x0 C. in the presence of a catalyst such as iron oxide (Japanese Patent Publication (KOKOKU) No. 6-38863(1994)); a method of removing halogenated aromatic compounds or the like from an exhaust gas or reducing amounts thereof by heat-treating at a temperature of 300 to 700xc2x0 C. in the presence of a catalyst containing iron oxide (Japanese Patent Application Laid-Open (KOAKI) No. 2-280816(1990)); a method of conducting the incineration of wastes under the coexistence of calcium carbonate particles and iron oxide particles in an incinerator (Japanese Patent Application Laid-Open (KOAKI) No. 8-82411(1996)); a method of adding a mixture of calcium hydroxide powder and activated carbon powder into exhaust gas flues in an incinerator (Japanese Patent Application Laid-Open (KOAKI) No. 11-63467(1999)); a method of introducing a blowing agent composed of calcium hydroxide and a porous inorganic oxide material (at least one material selected from the group consisting of clay minerals such as silicic acid, aluminum silicate, magnesium silicate, synthetic silicic acid, synthetic aluminum silicate, synthetic magnesium silicate, acid clay, activated clay, kaolin, bentonite, allophane and diatomaceous earth, and substances obtained by treating these clay minerals with acids to remove impurities such as aluminum and magnesium therefrom) into an exhaust gas passing through flues of an incinerator (Japanese Patent Application Laid-Open (KOAKI) No. 11-33343(1999)); a method of purifying an exhaust gas using a reactive calcium hydroxide-based cleaning agent (Japanese Patent No. 2602085); or the like.
In addition, as the methods of preliminarily mixing wastes with iron oxide or the like and then incinerating the wastes, there are known a method of burning combustible wastes at a temperature of not less than 850xc2x0 C. under the coexistence of an acid gas neutralizing agent, iron oxide particles and the like (Japanese Patent Application Laid-Open (KOAKI) No. 8-270924(1996)); and a method of burning wastes in an incinerator under the coexistence of ferric iron oxide hydroxide particles or iron oxide particles containing sulfur and sodium in not more than predetermined amounts (Japanese Patent Application Laid-Open (KOAKI) No. 9-89228(1997)).
However, although it has been desired to provide an process for treating an exhaust gas so as to inhibit the generation of dioxin from waste incinerators, the methods described in the above publications are still unsatisfactory.
Namely, in the method described in Japanese Patent Publication (KOKOKU) No. 6-38863(1994), poly-halogenated cycloalkyl compounds and poly-halogenated aromatic compounds in fly ash generated in an incinerator are decomposed by catalysts such as iron oxide, calcium carbonate or sodium carbonate in a fixed bed. However, this method fails to completely remove dioxin, and huge plant and equipment investment is required to construct a facility for converting the fly ash into unharmful substances, at a rear stage of the incinerator. Therefore, such a construction is almost impossible practically.
In the method described in Japanese Patent Application Laid-Open (KOAKI) No. 2-280816(1990), after ammonia is added to an exhaust gas containing halogenated aromatic compounds, the halogenated aromatic compounds are decomposed in the presence of an iron oxide-containing catalyst in a fixed bed. Therefore, the construction of such a complicated facility at a rear stage of the waste incinerator also requires huge plant and equipment investment.
In the method described in Japanese Patent Application Laid-Open (KOAKI) No. 2-82411(1990), iron oxide particles and calcium carbonate particles are caused to coexist with solid wastes in the incinerator. In this method, although the generation of dioxin in a combustion chamber of the incinerator is inhibited, the generation of dioxin and dioxin precursors cannot be sufficiently prevented in the course of cooling of the exhaust gas between a downstream portion subsequent to the combustion chamber and the dust collector.
In Japanese Patent Application Laid-Open (KOAKI) Nos. 11-63467(1999) and 11-33343(1999), and Japanese Patent No. 2602085, there are described techniques of absorbing harmful substances into activated carbon, porous inorganic oxide materials or the like. Therefore, the generation of dioxin cannot be sufficiently prevented. Further, huge treatment costs are required for converting the used absorbing agents into non-harmful substances.
In the methods described in Japanese Patent Application Laid-Open (KOAKI) Nos. 8-270924(1996) and 9-89228(1997), it is required to premix solid wastes with iron oxide particles, etc. Therefore, in view of the structure of an incinerator used therein, it is difficult to inhibit the generation of dioxin occurring either in the course of cooling of the exhaust gas between a downstream portion subsequent to the combustion chamber and the dust collector, or due to the memory effect upon low-temperature combustion caused in flues or dust collector at the start-up or shut-down of the intermittent operation-type incineration facilities.
Meanwhile, in the incineration method using iron oxide hydroxide particles or iron oxide particles containing sulfur or sodium in not more than a predetermined amount (Japanese Patent Application Laid-Open (KOAKI) No. 9-89228(1997)), the catalytic activity of the iron oxide hydroxide particles or iron oxide particles is sufficiently exhibited at a high temperature at which the incinerator is operated at a steady state. However, since the decomposition percentage at 250xc2x0 C. of monochlorobenzene is disadvantageously low, the generation of dioxin either in the course of cooling of the exhaust gas between a downstream portion subsequent to the combustion chamber and the dust collector, or due to the memory effect caused in flues or dust collector at the start-up or shut-down of the intermittent operation-type incineration facilities, cannot be sufficiently inhibited, as shown in Comparative Example 5 hereinafter.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by contacting an exhaust gas having a temperature of 150 to 500xc2x0 C. and existing between a downstream portion subsequent to a combustion chamber and a dust collector of an waste incinerator with an iron compound catalyst which comprises iron oxide particles or iron oxide hydroxide particles having an average particle size of 0.01 to 2.0 xcexcm, a phosphorus content of not more than 0.02% by weight, a sulfur content of not more than 0.6% by weight and a sodium content of not more than 0.5% by weight, and which has a catalytic activity capable of decomposing not less than 20% by weight of monochlorobenzene when 50 mg of iron oxide particles obtained by heat-treating said iron compound catalyst at 300xc2x0 C. for-60 minutes in air is instantaneously contacted with 5.0xc3x9710xe2x88x927 mol of monochlorobenzene at 300xc2x0 C. at a space velocity of 150,000 hxe2x88x921 in an inert gas atmosphere using a pulse catalytic reactor, or a composite catalyst composition containing the iron compound catalyst, it is possible to effectively inhibit the generation of dioxin. The present invention has been attained on the basis of this finding.
It is an object of the present invention to provide a process for treating a dioxin-containing exhaust gas discharged from intermittent operation-type incineration facilities such as mechanical batch incinerators and semi-continuous incinerators as well as continuous operation-type incineration facilities, which can inhibit the generation of dioxin either in the course of cooling of the exhaust gas between a downstream portion subsequent to a combustion chamber and a dust collector, or due to the memory effect upon the low-temperature combustion caused in flues or dust collector at the start-up or shut-down of the intermittent operation-type incineration facilities.
It is an object of the present invention to provide a composite catalyst composition for inhibiting the generation of dioxin in intermittent operation-type incineration facilities such as mechanical batch incinerators and semi-continuous incinerators as well as continuous operation-type incineration facilities, which can inhibit the generation of dioxin either in the course of cooling of the exhaust gas between a downstream portion subsequent to a combustion chamber and a dust collector, or due to the memory effect upon the low-temperature combustion caused in flues or dust collector at the start-up or shut-down of the intermittent operation-type incineration facilities.
To accomplish the aims, in a first aspect of the present invention, there is provided a process for treating a dioxin-containing exhaust gas comprising:
adding an iron compound catalyst comprising iron oxide particles or iron oxide hydroxide particles and having a catalytic activity capable of decomposing not less than 20% by weight of monochlorobenzene when 3.1xc3x9710xe2x88x924 mol (50 mg) of iron oxide particles obtained by heat-treating said iron compound catalyst at 300xc2x0 C. for 60 minutes in air is instantaneously contacted with 5.0xc3x9710xe2x88x927 mol of monochlorobenzene at 300xc2x0 C. at a space velocity of 150,000 hxe2x88x921 in an inert gas atmosphere using a pulse catalytic reactor, into an exhaust gas having a temperature of 150 to 500xc2x0 C. in the course of cooling between a downstream portion subsequent to a combustion chamber and a dust collector of a waste incinerator to contact the iron compound catalyst with the exhaust gas,
the amount of said iron compound catalyst added being 0.01 to 2.5% by weight based on the weight of dry wastes incinerated per hour, and said iron oxide particles or said iron oxide hydroxide particles having an average particle size of 0.01 to 2.0 xcexcm, a phosphorus content of not more than 0.02% by weight based on the weight of the particles, a sulfur content of not more than 0.6% by weight based on the weight of the particles and a sodium content of not more than 0.5% by weight based on the weight of the particles.
In a second aspect of the present invention, there is provided a process for treating a dioxin-containing exhaust gas comprising:
adding an iron compound catalyst comprising iron oxide particles or iron oxide hydroxide particles and having a catalytic activity capable of decomposing not less than 20% by weight of monochlorobenzene when 3.1xc3x9710xe2x88x924 mol of iron oxide particles obtained by heat-treating said iron compound catalyst at 300xc2x0 C. for 60 minutes in air is instantaneously contacted with 5.0xc3x9710xe2x88x927 mol of monochlorobenzene at 300xc2x0 C. at a space velocity of 150,000 hxe2x88x921 in an inert gas atmosphere using a pulse catalytic reactor, together with an acid gas neutralizing agent, into an exhaust gas having a temperature of 150 to 500xc2x0 C. in the course of cooling between a downstream portion subsequent to a combustion chamber and a dust collector of a waste incinerator to contact the iron compound catalyst and the acid gas neutralizing agent with the exhaust gas,
the total amount of said iron compound catalyst and said acid gas neutralizing agent added being 0.02 to 4.0% by weight based on the weight of dry wastes incinerated per hour, and said iron oxide particles or said iron oxide hydroxide particles having an average particle size of 0.01 to 2.0 xcexcm, a phosphorus content of not more than 0.02% by weight based on the weight of the particles, a sulfur content of not more than 0.6% by weight based on the weight of the particles and a sodium content of not more than 0.5% by weight based on the weight of the particles.
In a third aspect of the present invention, there is provided a process for treating a dioxin-containing exhaust gas comprising:
adding an iron compound catalyst comprising iron oxide particles or iron oxide hydroxide particles having a catalytic activity capable of decomposing not less than 20% by weight of monochlorobenzene when 3.1xc3x9710xe2x88x924 mol of iron oxide particles obtained by heat-treating said iron compound catalyst at 300xc2x0 C. for 60 minutes in air is instantaneously contacted with 5.0xc3x9710xe2x88x927 mol of monochlorobenzene at 300xc2x0 C. at a space velocity of 150,000 hxe2x88x921 in an inert gas atmosphere using a pulse catalytic reactor, together with an acid gas neutralizing agent and an activated carbon, into an exhaust gas having a temperature of 150 to 500xc2x0 C. in the course of cooling between a downstream portion subsequent to a combustion chamber and a dust collector of a waste incinerator to contact the iron compound catalyst, the acid gas neutralizing agent and the activated carbon with-the exhaust gas,
the total amount of said iron compound catalyst, said acid gas neutralizing agent and said activated carbon contacted being 0.02 to 5.0% by weight based on the weight of dry wastes incinerated per hour, and said iron oxide particles or said iron oxide hydroxide particles having an average particle size of 0.01 to 2.0 xcexcm, a phosphorus content of not more than 0.02% by weight based on the weight of the particles, a sulfur content of not more than 0.6% by weight based on the weight of the particles and a sodium content of not more than 0.5% by weight based on the weight of the particles.
In a fourth aspect of the present invention, there is provided a composite catalyst composition for inhibiting the generation of dioxin, comprising:
an iron compound comprising iron oxide particles or iron oxide hydroxide particles having an average particle size of 0.01 to 2.0 xcexcm, a phosphorus content of not more than 0.02% by weight based on the weight of the particles, a sulfur content of not more than 0.6% by weight based on the weight of the particles and a sodium content of not more than 0.5% by weight based on the weight of the particles; and
an acid gas neutralizing agent,
said composite catalyst composition having a catalytic activity capable of decomposing not less than 25% by weight of monochlorobenzene when 50 mg of a composite material of iron oxide particles obtained by heat-treating said iron compound catalyst at 300xc2x0 C. for 60 minutes in air, and said acid gas neutralizing agent is instantaneously contacted with 5.0xc3x9710xe2x88x927 mol of monochlorobenzene at 300xc2x0 C. at a space velocity of 150,000 hxe2x88x921 in an inert gas atmosphere using a pulse catalytic reactor.
In a fifth aspect of the present invention, there is provided a composite catalyst composition for inhibiting the generation of dioxin, comprising:
an iron compound comprising iron oxide particles or iron oxide hydroxide particles having an average particle size of 0.01 to 2.0 xcexcm, a phosphorus content of not more than 0.02% by weight based on the weight of the particles, a sulfur content of not more than 0.6% by weight based on the weight of the particles and a sodium content of not more than 0.5% by weight based on the weight of the particles;
an acid gas neutralizing agent; and
an activated carbon,
said composite catalyst composition having a catalytic activity capable of decomposing and adsorbing not less than 30% by weight of monochlorobenzene when 50 mg of a composite material of iron oxide particles obtained by heat-treating said iron compound catalyst at 300xc2x0 C. for 60 minutes in air, said acid gas neutralizing agent and said activated carbon is instantaneously contacted with 5.0xc3x9710xe2x88x927 mol of monochlorobenzene at 300xc2x0 C. at a space velocity of 150,000 hxe2x88x921 in an inert gas atmosphere using a pulse catalytic reactor.